Linux is a popular Open Source operating system developed originally to be similar to UNIX, but free to download and modify. There are countless different distributions of Linux, and it is being used in nearly every conceivable application.
It differs from BSD derivatives in several key ways. The most obvious and basic distinction is that Linux includes popular packages in a default install, where most BSDs consider anything but the base operating system as 'extra'. This has led to criticism of the core Linux distribution as being 'bloated'.